TDRSS DEMAND ACCESS SERVICE: APPLICATION OF ...

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4 Paper ID: 5a003 investigated and initiated implementation of a substantial change to the TDRSS MA return link service. Through an expansion of the MA service, users desiring such a capability will be able to obtain a continuously available MA return link to their spacecraft. Not only can the previous scheduling process be largely bypassed by users needing reduced operational complexity but, for the first time, it will be possible for a user spacecraft to initiate TDRSS return link communications without prior coordination by the user operations center. These changes will be implemented by expanding the number of MA return links available on the current TDRSS spacecraft and providing appropriate control/data routing equipment. 3.2 TECHNICAL APPROACH The combination of array beamforming and user signal PN spreading provides a high degree of separation among user signals within an MA return link demodulator. The TDRS architecture, if provided with sufficient beamformers and demodulators, could support a large number of return link users provided that individual user data rates are sufficiently low (hundreds of users at 1-10 kbps – depending on user duty cycle). In the past, costs associated with such expansion of the MA return service capability were prohibitive. The current WSC beamformers were implemented with 1980’s technology and each beamformer requires 30 circuit cards and occupies one full chassis of a 30 inch wide equipment rack. To understand the need for this level of complexity, consider that each beamformer receives a complete copy of the sampled data for all 30 array elements: 60 sampled channels (for I and Q) x 8 bits per sample x 8.5 Msps gives 4.08 Gbps processed by each beamformer. While the phase shifting and combining within each beamformer are not complicated operations, the data handling operations required to deal with this volume of data (both moving it among the individual circuit cards of a beamformer, and transporting it to all beamformers) are quite complex. Additionally, to meet redundancy requirements, each beamformer was further burdened by the need to perform a set of calculations (noise covariance matrix estimation) that were common to all the beamformers. The 1990’s have brought about substantial evolution in both digital processors and in data networking equipment. Taking advantage of these advancements, development of a new generation TDRSS beamformer has been initiated. The new beamformer implementation is expected to achieve a factor of five reduction in beamformer size and nearly a factor of ten reduction in per unit costs. Figure 3 provides an overview of the new beamformer architecture that emphasizes the signal multiplexing architecture. As a key first step in the development of the new beamformer concept, functionality was split between an Element Multiplexer/Correlator (EMC) unit and Individual Beamformer Units (IBUs). The EMC accepts the element data from the existing WSC A/D Quad Splitter, performs those calculations that were previously common to all beamformers, and distributes all beamforming data to the IBUs. Data distribution relies on a commercial off-the-shelf Network Transparent Switch (NTS). Data from six array elements is time division multiplexed together (giving five groups of six elements) then fed to the NTS for distribution to IBU Groups (IBUGs) containing five IBUs. Each IBUG contains a matching NTS receiver which demultiplexes the TDM data and distributes it to the IBUs.
1 2 • • • 30 A / D Q UADSPLITTER I,Q-1 I,Q-6 I,Q-25 I,Q-30 A/D Quad digitizes into 30 channels each @: 1. I &Q 2. 8 bits 3. Rate = 8.5 MHz EMC Node Board -1 (Mux) • • • EMC Node Board -5 (Mux) 1.0625 Gbps 1 5 Network Transparent Switch (NTS) ‘5’ to 15x’5’ (Creates 15 copies) EMC Node Board Muxes 6 Channel I,Q pairs = 6x(8+8+1)x8.5 MHz + Overhead = 1.0625 Gbps 1 2 15 5 x1.0625 Gbps • • • 5 5 EMC IBUG 5 To other IBUGs EMC NTS Provides 15 sets of the 5 1.0625 Gbps Data 5 Network Transparent Switch (NTS) ‘5’ to 5x’5’ (Creates 5 Copies) IBUG NTS Provides 5 sets of the 5 1.0625 Gbps Data 5 x1.0625 Gbps 5 1 Transition Module -1 (Serial to Parallel) Paper ID: 5a003 The NTS, in conjunction with modern high speed digital processing, greatly facilitates the beamformer implementation. A single EMC can support fifteen IBUGs for a total of 75 IBUs in the illustrated configuration. It is also possible, however, to use a single output from the EMC Network Transparent Switch transmitter to feed a distribution unit (based again on the same NTS switch) that regenerates the data for transmission to still more IBUGs. This flexible architecture can support hundreds of beamformers per SGLT – far more than are currently anticipated. Demand Access demodulators, currently being prototyped, will also be relatively inexpensive and much smaller than the current units and will also offer some performance improvements relative to the current WSC demodulators. Not only will the demodulator realize cost improvements due to advances in technology since the 1980s, the relatively large number of demodulators that will be produced will lower production costs by distributing non-recurring engineering costs over more units. One potential performance gain is in acquisition time. The ongoing NASA demodulator prototyping effort has centered around Charge Coupled Device (CCD) technology which allows rapid PN code correlation evaluations in the analog domain. Figure 4 illustrates the planned architecture of the new beamformers and demodulators within an SGLT of the WSC. The existing A/D Quad Splitters of the WSC have spare outputs available for connection of the new equipment. Existing beamformers and receivers within the WSC will not be affected by the service expansion. 5 5 • • • Transition Module -5 (Serial to Parallel) 1 • • 20 IBU -1 5 • 20 (Weight & Sum) 20 Parallel Lines Each 53.125 MHz 1 • • 20 IBU -5 5 • 20 (Weight & Sum) Transition Module Converts Serial to 20 bit Parallel Reduces each 1.0625 Gbps to 20 53.125 MHz lines Figure 3: The Beamformer Signal Multiplexing Architecture • • • IBU Combines all 30 channels; Output is a 6 MHz Signal
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